1. Field of the Invention
This invention is related to an image processing apparatus, an image reading apparatus, an image processing method, and a recording medium.
2. Description of Related Art
Techniques to convert low resolution multi-level image data to high resolution image data using a pixel block comprising a predetermined number of pixels have been proposed. Multi-level represents the number of tone levels larger than those an output pixel block can reproduce.
For example, when low resolution multi-level image data is converted to high resolution two-level image data, a tone conversion value is generated according to a dither threshold matrix, and tone conversion data is converted using the tone conversion value. In this regard, a method of image processing which rapidly generates high resolution two-level image data having granularity and resolution by referring to a pixel concentrated type dither threshold matrix was proposed (For example, Japanese Patent Laid-Open Publication No. 2002-185785). This method, by concentrating black pixels in the converted image data, prevents isolated black pixels from not reproduced in printing, thus enhances dot reproducibility in printing.
However, according to the above method, since the dither threshold matrix is used and a concentrating processing of black pixels is performed throughout the whole image data, a certain pattern is produced and texture noise easily occurs.